It has become essential to scrub gases produced in various hydrocarbon cracking processes to remove various contaminants. For example, flue gases generated in the production of ethylene are scrubbed with an aqueous sodium hydroxide solution to remove hydrogen sulfide, carbon monoxide and other impurities. As a result, the gaseous product stream is relatively clean, however, the treatment results in a liquid contaminated with the various impurities removed from the gaseous product stream.
A great deal of attention has been paid to treating the liquid effluent resulting from the caustic treatment of flue gases to provide an environmentally acceptable effluent for discharge into the environment.
One approach has been to oxidize sulfides and other inorganic sulfur acid salts in an alkaline solution to both thiosulfate ions and sulfate ions. The process typically used is referred to as a wet air oxidation (WAO) process in which gaseous oxygen in the form of fine bubbles is contacted with the spent caustic effluent in contacting columns for relatively long periods of time.
Chemical reagent oxidation (CRO) of sulfide-containing effluents has also been proposed as illustrated in U.S. Pat. No. 4,417,986 and U.S. Pat. No. 4,584,107 which teach the use of hydrogen peroxide and ozone in the treatment of spent caustic effluents.
Various other approaches to the treatment of water containing impurities have also been proposed. For example, the oxidizing of toxic compounds by using a combination of ozone, hydrogen peroxide and ultraviolet radiation has been proposed as illustrated in U.S. Pat. No. 4,849,114.
Similarly, mechanical separation methods employing ultrafiltration membranes have been used to separate solids and various high molecular weight compounds from effluents.
Combinations of the various processes have also been attempted with varying results.
However, a means to treat a spent caustic effluent to achieve an environmentally acceptable discharge product and regenerate the caustic solution has not yet been developed.